The present invention relates to a shadow mask or color selection electrode for a color television picture tube, as well as the support frame making it possible to stiffen or rigidify the mask.
A cathode ray tube for reproducing color television pictures generally comprises a glass envelope formed from a front panel or plate having a rectangular shape and extended by a skirt-like side wall, sealed to a conical part which narrows and which is terminated by the cylindrical or tubular neck with at its end three electron guns and having fitted to its outside horizontal and vertical electromagnetic deviators making it possible to sweep the phosphor screen.
This screen formed from phosphors of the three primary colors, red, blue and green, is deposited on the inner face of the front panel. In one type of color picture tube where the electron guns emit three parallel electron beams in the same horizontal plane, said screen is constituted by a repeated succession of three continuous bands of vertical phosphors having different colors R, G, B.
A color selection electrode constituted by a spherical or cylindrical metal surface 10 (FIG. 1) perforated by a large number of elongated rectangular or oblong openings or slots 11, called a shadow mask 12, is placed on the trajectory of the three electron beams in the vicinity of and substantially parallel to the screen, which has a shape identical to that of the surface 10. The effect of the shadow mask 12 is only to permit the passage of that part of each electron beam which is directed towards one of the bands of phosphors R, G and B, in such a way that the first beam is intended for the green bands G, a second beam for the blue bands B and a third beam for the red bands R, as a result of their different incidence angles at the location of slots 11. It should be noted that a single beam can have a section covering several slots 11 at once, as well as a circular area on the metal surface 10 surrounding them. Thus, approximately 80%, of the electrons of each beam is received by the metal surface 12, which is also called the mask fabric, which blocks their passage and absorbs the high kinetic energy thereof. This leads to a rapid heating of that part of surface 10 swept by the beams and which has a low thermal inertia.
Thus, during the manufacture of the tube, the shadow mask 12 must be removed and refitted several times and must also be able to withstand predetermined mechanical vibrations and shocks without undergoing any deformation or permanent displacement; it is generally supported by a rigid metal frame 16 (FIGS. 1 and 2) which is preferably made from a profile with an L-shaped cross-section and a thickness which considerably exceeds that of the mask (e.g. by 10 to 15 times). In the prior art tube constructions, described e.g. in FR-A-Nos. 1 470 260 and 1 486 675, the thickness of the metal sheet from which the metal surface 10 of mask 12 is made is generally between 100 and 200 micrometers, while that of the frame 16 is generally between 2 and 3 millimeters as a function of the screen dimensions. Therefore the thermal inertia of frame 16 is much higher and it is only heated much less slowly.
Thus, the shadow mask 12, which is permanently bombarded by electron beams and which is much thinner and weighs much less than the frame which is thick and heavy, is heated much more rapidly than the latter as soon as the tube is put into operation. The frame is only bombarded by electrons at the start and finish of each scanning line and field and only reaches its equlibrium temperature (when the heat supplied becomes equal to that dissipated) much later. Therefore mask 12 is subject to an expansion in its plane (in the radial direction) well before frame 16 starts to heat and expand. There is then swelling or convex curving of mask 12, whereof the center which is at the same time the apex, approaches the screen, and whereof the edges welded to the frame 16 remain in place through the latter, which is fixed to the skirt of the front panel with the aid of a conventional leaf or plate spring arrangement. This swelling of the shadow mask 12 leads to respective displacements of slots 11 of surface 10, which are purely axial in the center and which have decreasing axial components from the center towards the periphery (where they are initially zero) and radial components which increase from the center (where they are zero) to approximately midway between the center and the edge (where they reach their maximum value) and from there decrease up to the edge (where they are initially zero). This is diagrammatically illustrated in section in FIG. 1, where the broken line curve A shows the profile of a cold frame 16 and mask 12, while the dot - dash line curve B shows the profile of a hot mask 12 and a cold frame 16 bringing about the said swelling, indicated by a reduction in the radius of curvature of mask 12. The aforementioned displacement of slots 12 have the effect of displacing the axes of the beam portions, called index or chromogen strips, which pass through the same with respect to the vertical axes of the bands of phosphors R, G and B combined in juxtaposed triplets, so as to give rise to register losses or alignment faults, which are at their highest level in an annular area located roughly midway between the center and edge of mask 12.
This can lead to a relative reduction of the light intensity proportional to the surface of the bombarded phosphor (if the bands are separated by phosphor-free areas) or color purity faults, because a beam intended for a single phosphor will partly drop on to an adjacent band of another color.
After a given operating time of the tube, the frame 16 is also progressively heated by conduction, radiation, and electron bombardment, so that it also undergoes a thermal expansion. Frame 16 and mask 12 are generally made from the same material (rolled steel), so that they have the same thermal expansion coefficient. The expansion of frame 16, following that of mask 12, has the effect of, on the one hand, producing its previously noted swelling (accompanied by flattening with respect to the curve B of FIG. 3) and on the other hand increasing the distance between slots 11 thereof, i.e. radially displacing the same. This is diagrammatically illustrated in section in FIG. 2, whereof the broken line curve A (identical so that of FIG. 1) shows the profile of an assembly formed from cold frame 16 and mask 12, while the continuous line curve C shows a hot frame 16 and mask 12 assembly, i.e. which have reached the same equilibrium temperature. It can be seen that the area or extent of mask 12, as well as the distance between the pairs of parallel branches, have increased and that the radius of curvature of mask 12, following a brief reduction due to an initial swelling, becomes slightly larger than that which it had in the cold state. If the frame 16 is only suspended with the aid of spring plates, whose longitudinal axes are located in the same radial (transverse) plane and which are oriented substantially tangentially with respect to the circumference, frame 16 can expand in its plane without undergoing any axial displacement. This has the effect of stretching the spherical metal surface 10, in such a way that it spreads and is slightly flattened. Thus, mask 12 undergoes a slight axial displacement at the center, which increases with the radial distance, as well as a spreading in the radial direction, which has the effect of producing an increase in the spacing and to a lesser extent an increase in the width of slots 11. This causes register losses due to the spreading of slots 11 in the plane of expanded surface 10, which increase with the radial distance thereof with respect to the axis of the tube (i.e. with respect to the center of mask 12). It has been found that a supplementary displacement of the hot frame 16 - mask 12 assembly (profile C) in the direction of the screen following the tube axis has made it possible to compensate these register losses, because it makes it possible to substantially maintain the center of curvature of the surface of mask 12 in the intersection of the axis of the tube with the deviation plane normal to said axis. Such a forward axial displacement is illustrated by profile D (without frame) in FIG. 2 and has been obtained either by leaf springs oriented normally to the maximum deflection beams which the expansion of the frame pivots around two transverse folds or bends, in such a way that frame 16 moves towards the screen (cf. e.g. FR-A-No. 1 540 869) or with the aid of intermediate compensating members inserted between one end of the leaf spring and frame 16 formed from biplates, each constituted by two metal plates having different thermal expansion coefficients and which are superimposed and intimately joined together. These two solutions appear in the aforementioned FR-A-No. 1 486 675. These bimetallic compensating members, which can be intermediate between the spring and the frame (cf. also FR-A-Nos. 2 035 074 or 2 107 515) or constituted by biplate springs (cf. FR-A-Nos. 1 597 297 and 2 011 387) make it possible to compensate the alignment defect of slots 11 with the triplets R, G and B of the screen as a function of the temperature of frame 16 to which they are joined by welding. However, they do not intervene due to the initial swelling of mask 12, due to the significant difference between the respective thermal inertias (and weights) of the mask and the frame 16.
It has been possible to sigificantly reduce this swelling, together with other torsional effects exerted on the edges of the mask 12 by limiting the number of welding points joining the skirt of mask 12 to the belt of frame 16 which are parallel, or by providing the mask skirt or the frame belt with an alternating arrangement of radial cavities and projections limiting the contacting surfaces between the frame and skirt to areas surrounding the weld points. A supplementary reduction of this swelling has been obtained by reducing the radius of curvature of the metal surface compared with that of the conventional mask whereof the cold distance from the screen is substantially constant over its entire area. This measure also implies a variation in the spacing of the axes of the adjacent slots 11, as a function of the radial distance separating them from the center of mask 12, which increases the tolerances. It is pointed out that the tubes with parallel guns "in line" with lined screens and slotted masks are not very sensitive to register losses in the vertical direction, but are very sensitive thereto in the horizontal direction.
Apart from the overall swelling of the perforated mask, due to the slower heating of the frame, a localized swelling phenomenon has also been noted and is dependent on the content of the televised picture. For example, when the picture comprises two highly contrasted areas, whereof one is bright and the other dark, the number of electrons trapped by the area of the mask facing the former will be much greater than that received by the area of the mask facing the latter. It is obvious that the area which is bombarded most will rapidly reach a much higher temperature than that receiving few electrons; and due to the limited thickness and thermal conductivity of the mask, the respective temperatures of these two areas would remain different during a certain time interval if the contrasted picture persisted.
Color picture tubes usable more particularly for display purposes in informatics (e.g. in video) or in high definition television must have screens with finer or thinner phosphor bands and masks with less widely spaced slots (e.g. spacing reduced from 0.8 to 0.5 mm) than the presently available tubes. As a result there are greatly reduced tolerances with regards to the radial displacements of the slots, i.e. register faults due to swelling. Thus, a significant reduction (15 to 25 .mu.m) of the temporary swelling on starting up is indispensable.
It should also be noted that a flatter screen (i.e. with an increase in its radius of curvature) has the effect of increasing the register loss due to swelling. On combining the flat screen with an improved resolution, there is a further increase in the swelling reduction requirements.
The Applicant has found that it is possible to reduce the amplitude and duration of the temporary swelling of the mask by using a reduced thickness frame reinforced by at least one rib or bend, in order to have a mechanical rigidity comparable to that of a conventional thick frame with a L-shaped section.